Recently, a need exists for an electrophotographic image forming apparatus such as copiers, printers and facsimiles, which can produce high quality images at a high speed. In order to fulfill such a need, high speed image forming apparatuses capable of forming images at a speed of from 400 mm/sec to 1,700 mm/sec have been marketed.
However, when such high speed image formation is performed, image qualities tend to deteriorate, and in particular images having uneven glossiness tend to be formed. The reason therefor is considered to be that when high speed image formation is performed, the quantity of heat applied to fix toner images decreases, thereby making it impossible for a wax included in the toner to evenly exude from the inside of the toner. When the glossiness of a color image formed in a recording sheet varies, a serious quality problem in that the color tone, image density and color reproducibility of the image vary depending on the position of the image in the recording material sheet occurs.
Therefore, there is a need for producing images at a high speed while preventing the uneven glossiness problem. In order to fulfill the need, it will be necessary to efficiently transfer heat from a fixing device to a toner image while allowing the release agent included in the toner of the toner image to exude evenly and rapidly from the inside of the toner, so that the release agent is present on the surface of the toner image after the toner image is fixed.
In attempting to fulfill the need, a technique in that the fixing temperature is increased has been conventionally used. However, since the technique has drawbacks such that the inner temperature of the image forming apparatus seriously increases, the lives of members of the fixing device are shortened, and energy consumption of the image forming apparatus seriously increases (i.e., energy saving cannot be made), the technique is not proper.
Therefore, there is a need for a toner having such a good fixing property as to be able to be used for such high speed image formation and high speed fixation even when the fixing heat energy is relatively low.
There have been various proposals for toner in which a release agent included in the toner can easily exude from the inside of the toner.
For example, a technique in that the thermal property of a resin used for a toner is controlled to enhance the fixing property of the toner. However, when a resin having a low glass transition temperature is used for a toner, the high temperature preservability of the toner and the strength of fixed toner images tend to deteriorate. When a resin, which has a relatively low molecular weight and has a low softening point (F1/2 temperature), is used for a toner, a hot offset problem in that toner images are adhered to a fixing member, and/or a problem in that the glossiness of images of the toner increases to an extent such that it becomes difficult to control the glossiness of the toner images, are caused. Thus, until now, it is difficult to prepare toner which has a good combination of low temperature fixability, high temperature preservability and hot offset resistance by controlling the thermal property of the resin included in the toner.
There are several proposals in which polyester resins, which have relatively good low temperature fixability compared to styrene-acrylic resins which have been conventionally used as binder resins of toner, are used for toner. In addition, there are several proposals in which a specific non-olefin crystalline polymer, which can sharply melt at the glass transition temperature thereof, is used as a binder resin of toner. However, the molecular structure and molecular weight of the polymer are not investigated, and therefore exudation of a release agent from the inside of the toner is not optimized.
Further, in attempting to impart a good combination of low temperature fixability and high temperature preservability to toner, there are proposals in which a crystalline polyester resin and a non-crystalline polyester resin are used as binder resins of toner so as to achieve a sea-island structure in the toner, and proposals in which a resin including THF-insoluble components having a maximum endothermic peak in a specific temperature range in a differential scanning calorimetry (DSC) curve is used as a binder resin of toner. However, release agents included in the toners do not sufficiently exude fro the inside of the toners.
Furthermore, toners including a crystalline polyester in a relatively large amount have been proposed. When such toners are used for high speed image forming apparatuses, a toner filming problem in that a film of the toners is formed on an image bearing member, thereby deteriorating the image quality is often caused.
Conventional image forming apparatuses typically use a fixing device which has a fixing member (such as rollers and belts) and a pressing member and in which a recording material bearing a toner image thereon is fed through the nip between the heated fixing member and the pressing member to fix the toner image on the recording material. In this regard, the fixing member typically has a structure such that a heat storage layer made of a material having a high heat capacity and a high heat transportability is formed on a substrate, and a release layer is formed on the heat storage layer as the outermost layer. However, the materials used for the layers and properties of the materials are not optimized to an extent such that the uneven glossiness problem specific to high speed image forming apparatuses is solved by the image forming apparatuses.
Thus, the uneven glossiness problem is not yet solved by conventional techniques concerning toner and image forming apparatus.
For these reasons, the present inventor recognized that there is a need for an image forming apparatus which can produce high quality images at a relatively high speed without causing the uneven glossiness problem.